1. Field of the Invention
The present invention concerns a method to determine the design of a basic magnet of a magnetic resonance apparatus with at least one gradient coil system.
2. Description of the Prior Art
In magnetic resonance apparatuses that serve to generate image exposures of structures inside the body, a system of gradient magnetic fields is superimposed on the basic magnetic field for spatial coding of the image information. For this purpose, high currents are applied and removed again within the shortest possible time in the gradient coils serving for field generation. The conductors of the gradient coils are subjected to strong forces due to this rapid switching of the gradient coils in the strong magnetic field of the basic magnet.
The Lorentz forces that act on electrical charges in electromagnetic fields additively combine to produce large net forces. The forces accordingly acting on the gradient coil generate oscillations of the gradient coil system. These oscillations can cause noise as well as image artifacts and eddy current losses in the structure of the magnet or the cryostat fashioned with superconducting coils, with the result of a vaporization of the helium serving for low-temperature generation.
Therefore, in the design of the gradient coil system attempts have been made to take into account, as an additional consideration the minimization, of the net force that acts on this system when the gradient coils are located in volume in which the basic magnetic field is present. This additional limitation or mandatory condition leads to compromises in the specifications that are important, or possible, for the gradient coil system. For example, compromises must be made in the linearity, the rise time, the gradient amplitude and the scatter field.